Two opposing effects of calmodulin on microtubule assembly depend on the presence of microtubule-associated proteins.
The effect of bovine brain calmodulin on the assembly of pure bovine brain tubulin has been examined in the presence and absence of microtubule-associated proteins (MAPs). In the absence of MAPs, calmodulin enhances the rate and extent of polymerization of pure tubulin, probably by sequestering Ca2+ from tubulin since the effect is mimicked by ethylene glycol bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid and parvalbumin. From stoichiometric considerations, all 4 Ca2+ binding sites of calmodulin appear to participate in this effect. In the presence of MAPs, calmodulin confers increased Ca2+ sensitivity on the tubulin polymerization process, enhancing the inhibitory effect of Ca2+ on the rate and extent of assembly. The effect of calmodulin on the assembly of tubulin is dependent on the presence of Ca2+. The data suggest that calmodulin of both low (Ca1-22+.calmodulin) and high (Ca3-42+.calmodulin) Ca2+-induced inhibition of polymerization. Thus, calmodulin has dual and opposing actions on Ca2+ sensitivity of tubulin polymerization depending on the presence or absence of MAPs.
D000818:Animals; D001921:Brain / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D002135:Calcium-Binding Proteins / Q000502:physiology*; D002147:Calmodulin / Q000502:physiology*; D002417:Cattle; D004533:Egtazic Acid / Q000494:pharmacology; D007700:Kinetics; D008869:Microtubule-Associated Proteins; D008870:Microtubules / Q000187:drug effects / Q000648:ultrastructure*; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D011506:Proteins / Q000378:metabolism*; D014404:Tubulin / Q000378:metabolism
D002135:Calcium-Binding Proteins; D002147:Calmodulin; D008869:Microtubule-Associated Proteins; D009419:Nerve Tissue Proteins; D011506:Proteins; D014404:Tubulin; D004533:Egtazic Acid; D002118:Calcium